Spatial and Material Optimization for Novel Sustainable and Radio-Frequency-Friendly Micro-Homes

Abstract

The paper addresses the issue of designing a novel building that is sustainable, spatially optimized and also RF-friendly. The latter term is used to indicate that building materials guarantee good radio-signal penetration (in the range of frequencies of interest for commonly used wireless standards) and good reflection behavior indoors, thus enabling the use of innovative RF-based non-invasive human-monitoring systems that take advantage of RF-scattering-rich environments. The need for connectivity is becoming, more and more, a key inhabitant requirement. Nevertheless, so far, there are no examples of architectural designs that take into account the need for connectivity at the beginning of the design stage. As a consequence, once the house and the building material have been designed, then providing high-quality wireless connectivity services might become challenging. Therefore, the paper presents the design of a novel “house-concept” for vulnerable social groups that is made of a novel sustainable building material. Such a novel building material has been experimentally characterized in terms of dielectric properties. Simulation results have shown that such a material is not RF-friendly and solutions have been proposed to improve its RF propagation behavior and enable the use of a novel RF-sensing monitoring system.

1. Introduction

It is well-recognized that the use of information and communications technology (ICT) is, nowadays, a key element for innovative, more comfortable and energy-efficient houses. We are already surrounded by a multitude of connected devices that are able to provide innovative services and some of them aim to improve the energy efficiency of the house. Computing systems are increasingly embedded in buildings to regulate everything, from temperature and lighting to the right of access. With the Internet-of-Things (IoT) paradigm entering our lives and the increasing interest from companies and academia in smart homes and smart cities, it is becoming more and more important to envision the built environment as a connected, sensitive and responsive system [1]. However, designers and architects rarely consider connectivity outside of its functional paradigm: connectivity is provided simply by placing devices in the house, when both the spatial configuration and building material of the house have been already designed. However, often, design choices are not optimal from the radio-frequency propagation point of view and the task to provide high-quality wireless connectivity services becomes challenging. A novel design paradigm should be used, where the need for “connectivity” is taken into account at the beginning of the design stage. In the specific application scenario of this paper, the use of micro-houses as temporary accommodation for vulnerable social groups (i.e., people with health/social needs that are experiencing sanitary or other emergency situations), the use of such a new paradigm becomes even more important. Such micro-houses are sized for one person (11 sqm) and must have the following characteristics:

• Flexibility and modularity: they should be built in few days and then removed when the emergency is over;
• Made of eco-compatible materials, i.e., natural materials with a life cycle that is compatible with the environment: they are either recyclable or dispersible in the ambient without causing any damage;
• They should “transmit” to the vulnerable inhabitant a feeling of protection, wellbeing and safety.

To meet the last requirement, it is fundamental to equip the house with the innovative technological solutions able to provide non-invasive and remote monitoring of the health status or other useful information related to the inhabitant (for instance, information on his/her daily activity). More specifically, the monitoring system should be able to:

• Understand how the person moves inside the house. Such information could give important indications on the psychological status of the inhabitant and its evolution in time. For instance, it should be possible to understand if he/she spends most of the time on the bed, if he/she uses the kitchen or the washing machine or performs other daily activities that should become part of their life again;
• To alert, in real time, dangerous situations such as a fall, or intrusion of one or more persons when the house should be empty.

The term “non-invasive” refers to the fact that the system should:

• Preserve privacy;
• Assume that the inhabitant is not “collaborative”: the system is passive, in the sense that he/she does not have to carry any device (bracelet or smartphone).

First of all, the concept of a micro-house has attracted a lot of interest for different reasons and purposes, and many architects have worked on it. The scientific interest started with the pioneering indications of Alexander Klein on the theoretical model of the Existenzminimum in the 1930s [2]. The motivation to work on micro-houses has different objectives and motivations: geographical, cultural, emergency, social inclusion, design research, and the need for a house with low land consumption and environmental impact. However, all of them are centered on one concept: the protection of human dignity as well as his/her physical and psychological needs [3,4]. However, so far, many of the proposed solutions have mainly focused on the design of the spatial configuration, the building material and construction techniques. Thus far, no works have jointly addressed the above-mentioned requirements. Therefore, first of all, this paper presents the proposed innovative solution in terms of:

• Construction technique/spatial configuration;
• Building material;
• Monitoring system.

Then, we propose the use of RF-sensing device-free human-monitoring systems [5,6,7]. Such a type of system is based on the fact that a person inside a room modifies the propagation environment (by reflecting RF signals). There is a correlation between human activity and changes to the propagation channel. By properly catching this correlation, it is possible to use opportunistic RF signals (used for communication purposes) also to implement human-activity recognition systems. Most of the recent RF device-free systems use the so-called channel state information (CSI), which is strictly related to the multi-path propagation and frequency selectivity of the channel. Therefore, the RF propagation inside the house must be optimized both to provide normal connectivity to the user but also to implement such innovative RF-sensing systems; this optimization has an impact both on the building material and the spatial configuration of the house. Therefore, we first analyzed the RF propagation inside the building, characterizing sample materials and performing simulations; then, according to the results achieved, we proposed a novel solution both for the material and the spatial organization of the building elements. The paper is organized as follows: Section 2 presents the methodology; Section 3 shows the design of the micro-house, describing the specific choices in terms of construction technique, spatial configuration and building material; Section 4 focuses on the characterization of the interaction of the building material and the radio-frequency propagation and the final proposed solution; conclusions are drawn in Section 5.

2. Methodology

One of the main fields of application of the research on micro-architectures is temporary accommodation for vulnerable social groups. A prototype of the designed architectural structure is shown in Figure 1. The micro-architecture, which was named transit-House 2.0 Armònia, was made with an innovative construction system, named InDiesis, able to guarantee an excellent level of modularity and division of spaces. Moreover, it guarantees significant precision in the construction phase thanks to a positioning matrix made of perforated steel plates that also acts as a foundation [8]. We first designed the micro-architecture taking into account the requirement of flexibility (it should be possible to quickly build and collect it anywhere) and the specific needs of the users, who could be persons with social discomfort and in many cases not always collaborative. The spaces are organized and handled in different configurations so that a person could move into an ergonomic, flexible and limited-sized space. In this framework, the research on the domestic space should target both quality and essentiality of the space [9]. The integrated design of the Armònia housing module embodies the value of the architecture of the house as a support to social reintegration mixed with high adaptivity. The design involves disciplines specialized in different fields, with the objective to answer the challenge of integration and social inclusion, thus giving relevance to a particular type of user, consistent with human-centered design (HCD) [10,11,12]. From a humanistic point of view, the theme of the envelope was confronted. The house, historically conceived as a sort of protective skin, becomes an adaptive skin, malleable to different climatic situations, to the exchange between inside and outside, in search of a more usable, adaptable and customized domestic space for users with specific needs [13]. The field of investigation then shifts, from the architectural product (a material good) to the relationship between house and man’s daily actions (intangible asset), thus introducing the concept of “silent care and control” based on the virtual observation of the behaviour of the individual and his/her physical relationship with the place in which he/she lives. For this reason, the house, conceived and designed with particular attention to sustainability issues, also includes a non-invasive monitoring system based on the use of radio signals transmitted by a WiFi access point [5,6,7]. Figure 2 reports the concept of the designed micro-house. Such a monitoring system, which belongs to the class of so-called device-free RF sensing systems, allows to collect information on the way in which the resident moves around the house. By analyzing how much time he/she spends on the bed or in the kitchen and, in general, if typical actions of everyday life are carried out, it is possible to evaluate the health (physical and mental) status of the resident. In addition, thanks to the RF monitoring, it is possible to report dangerous situations in real time such as a possible fall or the intrusion of people inside the house. Therefore, we analyzed the RF behaviour of the designed building material and spatial configuration. The results of the analysis has provided feedback on how to improve the building material and the spatial configuration to make the micro-architecture also RF friendly.

3. Technical Construction Solutions and Sustainable Building Material

The In-Diesis technology, designed and patented, stands out from traditional prefabrication systems that generally use mono-blocks and single-function panels. The new construction technology is characterized as a dry-assembly building system, substantially composed of construction equipment divided into three multi-functional basic elements: foundation, wall and roof, as shown in Figure 3. Each of these components, in addition to the structural function, was designed to provide other specific functions to support integration with future technological and digital systems. In this process, the difficulties of the integration between architectural element (vertical brick) and the installation of technological systems, both traditional (i.e., electrical and water supply systems) and digital, were faced. One of the innovative elements is the structure of the minimum construction unit, which is a basic single component for the internal and external walls, with characteristics of ease of assembly and disassembly, being light weight (within 25 kg), assured handling without mechanical aids and competitiveness in construction time.

The new type of brick has a square section of 13 × 13 cm. Its peculiarity is in its vertical, no longer horizontal, form and it can cover in height and in a single solution the size of a habitable floor. This smart brick, called Diesis, guarantees great flexibility in the composition of spaces. The vertical brick is composed of a hollow core with a circular section covered by a tubular element in steel (the load-bearing Diesis) and PVC (the non-load-bearing Diesis) and an eco-compatible coating material, specifically designed from scratch and developed in the laboratory. The brick has a good technological and functional potentiality, as it can be specialized and adapted to the different needs of the technologies that can be integrated in the cylindrical cavity (see Figure 4). A qualitative aspect was always considered, which is the constraint to maintain the geometric figurative simplicity of the element within the complexity of the technical–constructive problems that this element has to solve. Therefore, the element acquires a double value: it is a generator of optimized spaces (partitions) and it is a bearer of a semantic and aesthetic value.

The coating material of the Diesis (see Figure 5) is the result of the research work conducted with the objective to add innovation through a sustainable material that has also characteristics of lightness, durability and aesthetic quality [14]. The scientific starting point was the experiments on Papercrete, a material based on paper, sand, aggregates and Portland cement, with which bricks and panels can be produced. The different laboratory tests on the mixtures for the new material initially considered the percentage values known in the composition of Papercrete, replacing the cement with hydrated lime and hydraulic lime.

The relation between sustainability and bio-architecture characterized the experimental phases on the material and the identification of raw materials to achieve the optimal composition of the miscellaneous requirements; the result is the definition of a mixture of aggregates, binders, dyes and fiber-reinforced products of natural and recycled origin which is in accordance with the premise of the research, according to the constraint of exploiting as much as possible raw materials with low environmental impact and scraps and derivatives of recycling, to develop a product that respects the above-mentioned principles. The transformation process from the aggregate mix to an “active” material takes place with the addition of water following a procedure similar to the processing of cementitious conglomerates up to casting in metal form-works. The casting phase initially revealed various critical issues both for the workability of the mixture and the appearance, after the demoulding, of various irregularities in the surface layer of the vertical brick. This is an unacceptable imperfection as the walls made up of the Diesis are designed to remain un-plastered. The search for material quality in terms of appearance, consistency and pleasantness to the touch led to further experimentation until these parameters were satisfied. The most important outcome of the research process is the fusion, in a single technological element, of constructive and functional aspects with aesthetic–perceptive ones.

4. Interaction between Building Materials and EM Waves

The interaction between the material and the spatial organization of the house with the EM waves was only partially studied [15]. Most of the past works only focused on the need to protect the inhabitants of the house from EM waves. The solution is the use of frequency-selective materials, i.e., materials that block some frequencies and let some other frequencies pass [16]. More recently, attention has been focused on the following issues [17]:

• The lack of penetration of EM waves, which reduces the possibility to use wireless devices inside the building. In particular, [18] reports a study commissioned by OFCOM (Office of Communications) on the use of metallic films in future energy-efficient buildings, which shield the building from EM waves, thus reducing the possibility to communicate inside the house.
• The optimization of EM waves’ indoor propagation to improve the coverage of wireless technologies that could be used indoors (for instance, WiFi). In particular, [19] faces the issue of improving WiFi propagation inside the house and thus WiFi signal coverage by introducing mirrors in some strategic points of the house.
• The optimization of the propagation of EM waves indoors for improving the performance of a novel class of services that are based on the passive use of the EM waves that are present inside the house: the so-called class of RF-sensing systems [7]. In particular, ref. [20] proposes the use of reconfigurable intelligent surfaces (RIS) to improve the performance of an activity-recognition system.

The above-mentioned issues are strongly related to the use of novel and sustainable building materials. Traditional building materials guarantee a good trade-off between signal penetration and the signal reflected inside the building.

Table 1. Permettivity and conductivity of some building material.

Material	Relative Permittivity	Conductivity (S/m)	Frequency (GHz)
concrete	5.31	0.0326	1–100
wood	1.99	0.0047	0.001–100
brick	3.75	0.038	1–10
glass	6.27	0.0043	0.1–100
metal	1	10	1–100

reports dielectric parameters of some of the most-common building materials; it can be noticed that concrete is characterized by a good conductivity (among these non-metal materials) but also high absorption. Brick walls are also characterized by good conductivity and lower absorption. Moreover, many old-style office environments are filled with metallic furniture (chest of drawers, lockers, dividing panels) which helps in increasing the level of reflections. On the other hand, wood-made houses and houses with many (normal) glass windows and with wood furniture, are expected to be characterized by a bad RF-reflection behavior, which is not optimal for WiFi (or similar technology propagation) nor for RF-sensing applications. In this paper, we focus on the characterization of the propagation inside the house, in particular, the characterization of the multi-path behavior of the materials.

4.1. EM Characterization of the Building Material

In this paper, radio-frequency friendly means a material that guarantees a good trade-off between these two characteristics: (i) a good penetration of RF signals at the frequency of current wireless systems (1–2 GHz); and (ii) a good multi-path propagation inside the building. These two characteristics are usually in conflict. As discussed in Section 4, traditional building materials, such as normal concrete and brick walls, show a good trade-off and, as a matter of fact, no specific issues have been raised so far.

Therefore, Section 4.1 investigates the RF propagation inside a building made of the novel sustainable material described in Section 3. In particular, the dielectric characterization of the novel material is performed through measurements of the scattering parameters. Figure 6 shows the experimental set-up. Measurements were made in a small anechoic chamber. We used a free-space method [21,22] to measure the scattering parameters. To perform the characterization of the dielectric properties of the material over a large range of frequencies, we used an ultra-wide-band (UWB) antenna—specifically, a flat monopole—with a dimension of 10 × 4 × 0.5 cm and an operative bandwidth that ranges from 1 to 3 GHz. From the measured scattering parameters over the overall range of frequencies, it is possible to estimate the relative permittivity at different carrier frequencies. We used the Nicolson–Ross–Weir method to estimate the relative permittivity at 2.4 GHz [23,24]. The NRW method is based on the transmission/reflection measurements of the object under test. The dielectric properties of materials are retrieved from their impedance and the wave velocities in the materials. The scattering parameters were collected using a $Rodhe\&Schwarz$ Vector Network Analyzer (VNA) ZNB4. The sample material was a perpendicular piece of wall with dimensions of 25 × 25 × 13 cm. In addition, the scattering parameters of a 5 cm wide cardboard sample and a 2 mm wide metallic sample were measured as reference. Figure 7 and Figure 8 show the measured scattering parameters (de-embedded in order to remove contributions not related to the medium under test) for the three samples. From Figure 7 and Figure 8, it is evident that the novel material has a behaviour similar to the cardboard, even if it has a slightly higher reflectivity.

The estimated relative permittivity at 2.4 GHz of the novel building material is: ${ϵ}^{{}^{\prime }}=2$ and ${ϵ}^{{}^{\prime \prime }}=0.6$.

4.2. Two-Ray Tracing Simulator

To characterize the channel impulse response of a room whose walls are made of the novel building material, we developed a simulator that, given the electrical properties of the building material, is able to quantify the level of multi-path propagation due to reflections. The simulator uses the ray-tracing model for the EM propagation [25]. We considered a rectangular 9 × 5 m room, where some furniture is present. In particular, we assumed that two small wood tables were located inside the room. Walls were made of a uniform material, such a concrete or another material. We assumed to transmit WiFi signals at 2.4 GHz and with 20 MHz of bandwidth (we could not distinguish replicas that are closer than 50 ns). Figure 9 shows the channel impulse response when the building material is concrete. As a reference, the channel impulse response in the extreme case in which the walls are made of metallic material was also estimated (see Figure 10). Figure 11 shows the channel impulse response when the novel building material is used.

Table 2. Dielectric properties of compared materials.

Material	${\mathit{ϵ}}^{{}^{\prime }}$	${\mathit{ϵ}}^{{}^{\prime \prime }}$
metal	1	${10}^8$
cardboard	1.8	0.8
concrete	5.31	0.95
novel material	2	0.6

shows the dielectric properties of the considered materials. It is worth noting that a 2D simulator was used, i.e., the reflections occur in a plane. Therefore, we did not obtain a fine estimation of the channel impulse response. However, for the purpose of this paper, which is a preliminary characterization of the multi-path propagation, the estimated coarse channel response provides meaningful information. Roughly, the maximum delay spread in the case of metallic walls is of the order of 1 µs. As expected, the delay spread in the case of concrete is much lower, about 140 ns. The new sustainable building material is characterized by higher losses, the number of reflected replicas received with a sufficient power level (we set the sensitivity of the receiver to $-90$ dB) is less and the delay spread is around 80 ns. Therefore, in such an environment, RF-sensing monitoring systems are expected to have worst performance than in the case of concrete, regarding the multi-path propagation the source of information. For the same reason, the WiFi coverage inside the room is negatively affected, as few multi-paths means that the connectivity is mainly dependent from the presence of line-of-sight (LOS) conditions.

5. RF-Friendly Building Material and Spatial Optimization

Results presented in Section 4 suggest that the material should be modified to make it more RF friendly. We wondered if we could improve the electro-magnetic behaviour by drugging the material. As drugging materials, we considered the materials whose main characteristics are reported in

Table 3. Properties of drugging materials.

Material	${\mathit{ϵ}}^{{}^{\prime }}$	${\mathit{ϵ}}^{{}^{\prime \prime }}$	Density kg/m3
graphite	23	19.5	2200
carbon black	8	2	1700

, graphite and carbon black. The dielectric constant of the miscellaneous of these two materials can be approximated as follows [26]:

$${ϵ}_{rm}^{{}^{\prime }}={{ϵ}_{r1}^{{}^{\prime }}}^{\frac{V_1}{V}}{{ϵ}_{r2}^{{}^{\prime }}}^{\frac{V_2}{V}}$$

(1)

$$tan{\delta }_m=\frac{V_1}{V}tan{\delta }_1+\frac{V_2}{V}tan{\delta }_2$$

(2)

where: ${ϵ}_{r1}^{{}^{\prime }}$, ${ϵ}_{r1}^{{}^{\prime }}$, $tan{\delta }_1$ and $tan{\delta }_2$ are the real part of the permittivity and the tangent loss of the two materials of the miscellaneousrespectively; $V_1$ and $V_2$ are the volumes of the two materials; $V=V_1+V_2$, ${ϵ}_{rm}^{{}^{\prime }}$ and $tan{\delta }_m$ are the real part of the permittivity and the tangent loss of the miscellaneous, respectively. Considering that the density of the novel material is 702.4 kg/m${}^3$, assuming a percentage in weight of $30\%$ for the drugging material, we obtain the following new dielectric properties for the miscellaneous material: ${ϵ}^{{}^{\prime }}=2.68$ and ${ϵ}^{"}=0.98$, when the graphite is used; ${ϵ}^{{}^{\prime }}=2.46$ and ${ϵ}^{"}=0.94$, when the carbon black is used.

A lower percentage in weight would result in a not-significant change in the dielectric properties. Figure 12 and Figure 13 show the impulse response of the channel when drugging materials graphite and carbon black are used, respectively. It can be observed that, in both cases, the impulse response of the miscellaneous material is now more similar to the one of the traditional concrete, thus showing better performance with respect to the non-drugged Diesis brick. It is worth noting that such a drugging process causes a strong change in the colour of the building material. Figure 14 shows samples of the original material and the material drugged with carbon black and graphite. From the point of view of the research on the architectural homogeneity of the interior spaces, such a chromatic alteration affects the previously stated goals, i.e., to create domestic spaces designed with simple elements to achieve a spatial balance, without inserting variations in shapes and objects. However, it was decided to accept such a change in the colour as a positive effect of a non-negotiable cause. The solution is identified in the integration of the “element of exception” in the rooms of the house: a piece of furniture such as a technological totem positioned within the spaces, visible and spatially defined, which has both a functional and aesthetic value.

6. Conclusions

The paper presents the first attempt to make a joint design of a novel building that is sustainable, spatially optimized, but also RF-friendly. The paper first presented the analysis of the dielectric properties of a novel building material, the Diesis brick. Moreover, the channel impulse response was estimated through simulations. Such an analysis revealed that, in a building made of such a material, radio-frequency signals, such as WiFi signals, are weakly reflected. Therefore, we proposed the use of materials that contains a percentage of graphite or carbon black. With a percentage of $30\%$, we obtained a better channel response to the RF signal. In this preliminary phase of optimization, we assessed the indoor RF propagation of the original material and the drugged material through simulations, which already provide us with useful indications. Of course, a more accurate optimization can be performed by using an experimental assessment. As perspective, we plan to build a prototype house with the proposed “drugged” material to experimentally assess the performance of the RF sensing monitoring systems. Moreover, it must be outlined that the achieved materials have two main drawbacks: (i) they are characterized by a strong change in color; (ii) they lose their workability, as the introduction of the proposed material has an impact on the compactness of the miscellaneous during the manufacturing process of the brick. The change in the color calls for a further step of architectural spatial optimization. The final solution foresees a piece of furniture, which could be a technological totem positioned within the spaces, which is visible and spatially defined and made of the designed material. The further optimization considering workability might lead to different choices for the drugging material and this will be subject of future research activity.

The paper presented an experience of the joint optimization of the spatial organization and RF connectivity of a house. It is worth outlining that, in a future where sensing will become pervasive and houses more and more smart, the capability to use materials and architectural elements able to improve the performance of wireless communication services will be fundamental.